Manufacture of electrical coils

ABSTRACT

A lacquer insulated metal foil coil has continuous interturn and end face insulation the latter being produced from lacquer exuded from between the turns in a heating step.

United States Patent mi 11] 3,750,275 Kay et al. [4 1 Aug. 7, 1973MANUFACTURE OF ELECTRICAL COILS [56] References Cited [75] Inventors:Albert Ernest Kay; Arthur Wellesley UNITED S S ENTS Stewart Clark, bothof London, 3,518,755 7/1970 Olson 29/605 England 3,545,078 12/1970Lightner 29/605 [73] Assignee: ghlllllilalrilcllum Foils Limited,London, i y Examiner chafles w Lanham g Assistant Examiner-Carl E. Hall[22] Filed: Aug. 23, 1971 Attorney-Holcombe, Wetherill & Brisebois 2i Al. No.1 174 203 I 1 pp 57 ABSTRACT A lacquer insulated metal foil coilhas continuous in- [30] Foreign Priority Data terturn and end faceinsulation the latter being pro- Aug. 28, I970 Great Britain 41,5l3/70duced from lacquer cXuded from between the turns in a heating step. [52]U.S. Cl 29/605, 29/2542, 336/205,

336/206 2 Claims, No Drawings [51] Int. Cl. H0lf 7/06 [58] Field oiSearch 29/605, 25.42;

MANUFACTURE OF ELECTRICAL COlLS This invention is concerned withimprovements in and relating to the manufacture of electrical coils frommetal foil, which coils are suitable for electro-magnets, solenoids orother inductors. In particular it concerns the production of coilsconsisting of ribbons of foil coated with an adherent insulatinglacquer.

The method of production of such coils by slitting a wide web ofaluminium foil into ribbons of suitable width is well known. A normalmethod of providing the interturn insulation is by interleaving theribbon with a ribbon of paper or other non-conducting film during theoperation of winding the coil. It is well known that the ribbon ofnon-conductor should be a little wider than the metal foil and should bepositioned-so that there is an overhang on both edges. By thisprecaution it is possible to avoid the danger of short circuits betweenadjacent turns due to deformation of the edges of the metal strip. Whenuse is made of an adherent insulating lacquer to separate the turns,overhanging edges cannot be obtained from the slitting machine.Additional operations, known as edge conditioning, have to be performedto eliminate edge deformation or rags and to provide a longer pathbetween the conductors than the interturn gap due to the thickness ofthe lacquer. This effect can be provided by a process of etching inwhich a liquid is used which will dissolve the metal without affectingthe lacquer. Such etching processes have been described for themanufacture of capacitors and solenoids. The etching processes sufferfrom the drawback, besides being an additional manufacturing operation,that they are wet processes and require thorough washing followed bydrying to ensure the complete removal of strong reagents used in theetch and all water which might tend to set up corrosion of the metalconductor within the coil. In addition, very careful control of themetal removal process is required to ensure that it is uniform and thatthe etchant does not penetrate between the metal and the lacquer, whichundercutting weakens the overhanging lacquer. There is a furtherdifficulty that the width of the conductor, and therefore its crosssectional area, is dependent for its accuracy on the control of theetch, which is much less accurate in establishing the width of theribbon than the slitting machine.

The present invention provides a process for the manufacture of anelectrical coil including the steps of coating a metal foil with anadherent insulating lacquer, slitting the coated foil'and winding to acoil, in which process there is used as insulating lacquer a lacquercomposition of a thermosetting nature andcapable of undergoing by theaction of heat a curing or further curing whereby the lacquer is firstrendered fluid and finally sets, the tension applied in windingthe coilis sufficient to cause or allow the fluid lacquer to exude from betweenthe turns of'the coil, and the wound coil is subjected to such a degreeof temperature and for such a time that exudation and setting produce afinal coil in which the interturn insulating effect is preserved andinaddition an edge insulation is provided. When the coil is of otherthancircular shape, the windingtension may not give a uniform interturnpressure at all parts of the coil andthis pressure may have to beaugmented by the use of a jig with correctly positioned pressure platesduring the curing phase.

By this process the above difficulties can be overv come. The lacquercan be applied by any of the established methods of coating and theribbons of coated foil can be slit to the desired width by any wellknown slitting machine taking the usual care to produce an edge as freefrom deformation as possible and with the edge of the lacquer layerlaying flush with the edge of the metal conductor. It is howevernecessary to control the tension applied to the conductor as it is beingwound within the limits described.

For this purpose it is not necessary to depart from the ordinary processof winding using, for example, the Kampf reeling machine normallyemployed in the aluminium foil converting industry. All that is requiredis that in such a winding operation the tension be carefully maintainedthroughout. The control of the wander of the turns, i.e., the variationin the relative positions of the superposed edges, should also becarefully observed. Where, in spite of such control, the degree ofuniformity at the edge of the coil is insufficient to produce asatisfactory result from the use of the process of the invention, it maybe necessary to face up the edge in some conventional manner of amechanical or chemical nature.

When the coil has been wound to the desired size, and terminationsattached as required, it is heated in an oven. Under the influence ofthe heat, the lacquer or apart of it liquefies and expands, or merelyliquefies, and the pressure produced as a result of the tension ofwinding the coil, or by the expansion of the lacquer, causes thelacquer, or part of it, to flow and to be exuded beyond the edge of theconductor. Furthermore, the lacquer being of a thermosetting nature, bythe continued' action of the heat it reacts and becomes rigid. When thecoil is removed from the oven and allowed to cool, it has become a solidmass held together by the adhesive nature of the thermoset lacquer whichforms the insulating layer between the turns. Moreover the faces of thecoil are now usually covered by a continuous film formed by that part ofthe lacquer which was exuded. By this means the conducting turns areseparated from eachother and protected from outside influences.

It will be understood that no intermediate process between slitting thefoil' into ribbons and winding the desired coil is required, exceptingonly in the circumstances described above where it is necessary tocorrect deficiencies in the winding technique. Further the width of themetal conductor remains as produced by the setting of the slittingknives or as resulting from the facing operation.

In a modification of the process of the invention, instead of thelacquer composition being of a thermosetting nature it is of athermoplastic nature, the tension applied in winding the coil issufficient to cause or allow the fluid lacquer to exude from between theturns of thecoil, and the wound coil is subjected to such a degree oftemperature and for such a time that by exudation, then followed bycooling, a final coil is produced in which the interturn insulatingeffect is preserved and in addition an edge insulation'is provided.

In this case the resulting coilis of more limited applicability since itcan only be used at temperatures so far below the melting point of thethermoplastic resin that no movementwithin the coil takes place inservice.

Thepraetice of'thisinvention obviously requires that the originallacquer film. be produced in a dry and nontackycondition' and that, inthe case of a thermosetting lacquer, it can later be irreversiblyreacted by heat on one occasion. Within certain limits, to allow forexudation, the quantity of lacquer applied is in inverse proportion tothe width of the ribbon used to form the coil, the less the width of theribbon the greater the thickness of the lacquer required. For ribbons ofmore than a certain width, however, this proportionality is no longerimportant. The quantity of lacquer required is in direct proportion tothe thickness of the ribbon and is also markedly affected by theprecision of the alignment of the convolutions one upon the other.Furthermore the tension applied to the ribbon by the coil windingmachine must be sufficient to produce a minimum interturn pressurewithout which the lacquer will not be exuded. Lastly, also in the caseof a thermosetting lacquer the temperature must be above a certainminimum to ensure that the time available for flow to take place issuitably related to the gel time of the lacquer.

The type of lacquers which may be used in this invention is very varied.Obviously the dry lacquer solids must form a layer which is attached tothe metal sufficiently strongly to be wound, unwound and slit withoutdamage at a convenient temperature. At a higher temperature the resinsolids must melt to a liquid whose viscosity is sufficient to allow itto flow under the pressure applied to it. it is also useful if thelacquer is of such a type that it can form two phases on melting. One ofthe phases, being more mobile than the other, exudes more rapidlyleaving the other to maintain the separation between the adjacentconvolutions. One component of the lacquer may be solid from the classof finely ground thermally inert non-conducting materials. Such a solidmaterial remaining between the convolutions maintains the integrity ofthe insulation between them. In the case of a thermosetting lacquer, ata still higher temperature, reaction must commence within the moltenlacquer solids leading to a rise in viscosity and gelation consequentupon cross-linking. The range between these two temperatures may betermed the critical temperature range. There is thus a relationshipbetween the viscosity and the appliedpressure over the criticaltemperature range. The amount by which the lacquer will move will alsodepend on the length of time that it remains within the criticaltemperature range. The period can be lengthened if required by reducingthe degree of heating of the coil but cannot be reduced below theminimum time required by the coil to absorb heat. Since cross-linkingmay not occur instantaneously on reaching the cure initiationtemperature, the time withinthe critical temperature range may be variedby choosing different properties of curing agent or by choosing more orless reactive curing agents.

It follows from the above that in the practice of this invention. it ispossible to fit the mechanics, i.e., the winding tension, the coatingweight and the width, to the lacquer available or to accept given valuesof these and to select a lacquer to suit, or both approaches may bepartially adopted.

Lacquer systems which are useful include solid epichlorhydrin resinsblended with solid curing agents, e.g., dicyandiamide, phenol, cresol orsimilar aromatic hydroxylated molecules after resinification withformaldehyde, or similar well known urea-formaldehyde ormelamine-formaldehyde resins: solid polyhydroxy alcohols which reactwith isocyanates blocked so that they remain inactive below a certaintemperature may also be used. Other well known lacquer systems are alsosuitable. Application is usually from solvent solution but other methodse.g., hot melt, electrostatic deposition or powder coating may be usedprovided they meet the requirements given above.

T he following Examples illustrate the invention; (R) means RegisteredTrade Mark:

EXAMPLE 1 The winding of a solenoid coil is constructed from fullyannealed commercial purity (99.2 percent) aluminium foil which has beenreduced by rolling to a nominal thickness of 0.015 mm. A web of thismaterial, approximately 720 mm wide, is coated with a solution of:

Epoxy Resin Shell Epikote'fl 1001 37 parts by weight Phenolic ResinBakelite" R 18973 11 parts by weight in Methyl Ethyl ketone (m.e.k.)methylated spirit in the ratio 2:1 52 parts by weight.

Before application, the viscosity of this solution was adjusted withadditional solvent so that a dry weight of 4.3 grams/sq. metre remainedon the foil surface after evaporation of the solvent. The coating anddrying were carried out on a roller application coating machine with asteam heated drying drum whose temperature did not exceed C. The coatedweb was reeled up and conveyed to store in readiness for the nextoperation. Tests on this foil showed that the temperature at which thecoating began to adhere to plain foil was 63C. There was no danger inkeeping the reel in store at normal room temperature. When heated to Cfor one hour the coating lost all solubility in m.e.k. and could not beremoved from the foil by pulling with Scotch Tape. A lap joint madebetween the coated foil and plain foil at 190C with a pressure of 0.04kg/sq. cm withstood a sheer load of 200 g. on a 4 sq.cm area at atemperature of 190C. This web of coated foil was slit and wound into aset of 26 mm wide reels on tmm internal diameter cores until a finaldiameter of 108 mm was reached. This was done on a Kampf reeling machineas is usual in the aluminium foil converting industry. Care was taken tomaintain the tension in the web. After attaching a termination andsecuring the end, the reels were placed in an oven heated to 200C forone hour while supported on a bar with their axes horizontal. Whenremoved and cooled, the end faces of the reels were seen to be coatedwith the hardened exudate from the lacquer. Some of the coils weresectioned and it was found that the resin coating between theconvolutions was 3 to 4 micrometres thick and the minimum thickness ofexudate over the most protruding convolution was 6 micrometres. Theelectrical resistance of the coils was 17.07 ohms. This was very similarto the value of 17.86 ohms. obtained by rewinding one of the reels fromtheset described and interleaving it with insulating paper whichoverhung 2 mm. on either side of the foil conductor as described in theprior art. The coils were suitable for installation without furthertreatment.

EXAMPLE 2 In another application a solenoid coil was made for anautomobile switch. The winding is made from commercial purity aluminiumas-before but of nominal thickness 0.018 mm. A web of this material,approximately 180 mm wide, is coated with the solvent solution of thefollowing resinous materials:

Epoxy resin Shell Epikote" 1001 37 parts by weight Heat hardeningphenolic resin Bakelite R 18973 5.5 parts by weight Non-heat hardeningphenolic resin Bakelite" R 5.5 parts by weight This was dissolved in thesolvent mixture described in Example 1 so that a dry weight of 3.66gram/sq. metre remained on the foil surface after evaporation of thesolvent. The coating and drying 'were carried out on a gravure rollerapplication machine with an electrically heated drying plate whosetemperature did not exceed 140C. The coated reel was then stored untilrequired for the next operation. Tests carried out as in Example 1showed similar performance. The web of coated foil was now slit intoreels 26 mm wide on 58 mm diameter cores which were transferred to acoil winding machine. The ribbon of coated foil was then wound onto abrass core of 18 mm diameter until the winding diameter became 34 mm.This required 27.42 m of coated foil. Care was taken to maintain tensionin the web and to control the wander. A termination was attached to theend of the winding and the end secured by suitable adhesive tape. Aftertreatment in an oven at 200C for one hour supported by a bar with itsaxis horizontal, the coil was allowed to cool. The end faces were foundto be completely covered with the hardened exudate. The resistance ofthe winding, measured between the termination and the brass core whichis in contact with the uncoated side of the first convolution wound, wasfound to be 1.603 ohm, being about 95 percent of the theoretical 1.681ohms calculated from the length, width and nominal thickness of thealuminium. When the coil was sectioned it was found that the averagethickness of insulation between convolutions was 3 pm. The windingshowed a degree of wandering giving a mean step height (i.e., theprotuberance of the edge of any coil turn beyond the innermost coil turnedge) of 60 pm. The edge of the convolutions were completely covered bya resinous layer 100 to 150 pm thick with a minimum of 40 am over themost protruding convolutions.

. EXAMPLE 3 A solenoid coil is concerned whose ampere-turns figure wasrequired to bel935 on a 12 volt supply. Calculation showed that 333turns of ribbon conductor 26 mm wide by 0.015 mm thick was required. Aribbon coated foil prepared in a manner similar to that described inExample 2 was taken to a coil winding machine and the required number ofturns wound on to a 22.5 mm o.d. 2.75 mm wall thickness core using greatcare to maintain tension in the strip and to control the wander of theribbon during winding. A termination was attached to the end of theconductor ribbon and the end secured with suitable adhesive tape. A jigwas made which consisted of two parallel strips ofpolytetrafluoroethylene attached to a base so that they were heldvertically with their long edge horizontal and separated by 22.5 mmequal to the o.d. of the winding core used. The upper edges of thestrips had previously been chamfered off at 45 so that a knife edgeexisted on the inner edges of the strips A number of coils prepared asdescribed above were laid on this jig with their axes vertical and thecores centrallised between the two PTFE strips. The coils did not toucheach other. In one experimental run a block was inserted to carry theweight of the core and in another this was omitted. The jig carrying thecoils was placed in an oven at 200C for 1 hour. When'removed and cooledthe coils were found to be covered on both faces with the hardenedexudate. The resistances of the coils were typically 2.05 or 2.04 ohmswell within the tolerance allowed and the coils were suitable for theirdesigned duty. On sectioning sample coils the thickness of insulationwas found to be approximately 3 pm. The mean step height was 33 pm andthe average thickness of the exudate was pm with a minimum of 5 pm overthe most protuberant convolution.

EXAMPLE 4 It will have been revealed in the previous Examples that it ispreferred that the thickness of the metal ribbon and the thickness ofthe initial coating should be in the approximate ratio of 4 to l orless. This assures an adequate supply of exuded lacquer to cover theedge faces of the coils. in one case it was necessary to use a ribbon offoil 0.096 mm thick carrying a fully thermoset insulating lacquercoating on one side. For the purpose of this invention it would havebeen preferred to apply approximately 25 g/sq.m of the coating describedin Example 1 to the other side of the foil ribbon. The limitations ofthe coating machinery available were such that it was not possible toapply more than 6.75 g/sq.m. of the coating. Since this was less thanwas desired, the following procedure was adopted to make the best use ofthe quantity available, by reducing the turn to turn misalignment to theminimum. The coated web was slit to 27 mm width and this ribbon taken tothe coil winding machine and wound into a coil on 30 mm diameter coretaking care to maintain tension and to control the wander to within 10.5 mm. The end of the winding was secured with suitable insulating tapeand the coil mounted in a lathe. It was then machined so that both faceswere smooth and 26 mm apart. The faces were then brushed with a stiffbrush to remove as many rags as possible and then washed over withTuckers Etch which remained in contact with the surface for about 30seconds before being washed off. Since the contact time had been soshort there was no penetration, into the coil and drying was simple.Examination of the surface showed that the convolutions were clear ofeach other. The coil was then placed in an oven at 200C for one hoursupported with its axis horizontal on a bar through the core. Whenremoved and cooled, the edge faces were found to be protected by a thinfilm ofexudate. On sectioning this was found to cover the edges of theconvolutions to a depth of about 5 micrometres. In this coil theinterturn insulation was provided by the thermoset insulating lacquermentioned above. It may appear that the advantage of the invention islost by the need to use an etch but this is not so since the amount ofetching and the time spent doing it are much reduced compared with theprior art. Rather is this regarded as a method whereby the process ofthis invention can be extended to be useful when the coating thicknessis limited and the foil or strip is thick.

EXAMPLE 5 The method of preparation described in the foregoing examplescan also be applied when the lacquer used does not possess thermosettingproperties. A ribbon of foil 0.10 mm nominal thickness and 30 mm wide,was coated with a tough polyamide resin, Versalon" (R) l 175, suppliedby Cray Valley Products Ltd.. This is a thermoplastic material with aBall & Ring softening point of l70-l80C. The material which is suppliedin granular form was dissolved at I g/l in a mixture of toluene andethyl alcohol as recommended by the makers. This solution was coatedonto the ribbon of foil using a wire-wound spreader bar and the solventevaporated to leave a dry coating weight of l2.0 g/sq.m.. After cooling,the ribbon of foil was reeled up ready for the next process which was towind a coil on a 30 mm diameter core. Winding tension was applied sothat the aluminium was elastically stretched and a firm dense windingproduced, and the end firmly secured to retain this tention. The coilwas then mounted in a lathe and machined so that both faces were smoothand 26 mm apart. The faces were then brushed to remove as many rags aspossible and then washed over with Tuckers Etch. The faces were thenrinsed and dried. Examination of the surfaces showed that theconvolutions were clear of each other. The coil was then heated in anoven until it reached a temperature of 246C being supported onpolytetrafluorethylene knife edges as previously described. The coil wasthen allowed to cool without disturbance. When it was cold it was seenthat the surfaces were covered with a thin film of exuded Versalon 1175.The coil was suitable for use on duties which did not cause itstemperature to rise above ambient.

We claim:

1. A process for the manufacture of an electrical coil,

comprising the steps of l. preparing a web of metal foil coated with asolid dry coating of a lacquer that is initially heatmcltable andfinally heat-curable to form a thermoset lacquer;

2. longitudinally slitting said foil having said dry coating to producea ribbon having cut longitudinal edges, said coating extending to saidedges;

3. uniformly winding said ribbon under tension to form a coil withclosely superposed edges;

4. attaching electrical terminations to said coil; and

5. subjecting said coil to controlled heat; whereby sufficient of saidlacquer is exuded from between said turns on melting to producecontinuous end face insulation for the coil while leaving adequatelacquer for inter-turn insulation, said heating being continued to curesaid lacquer.

2. A process as claimed in claim 1, in which the end faces of the woundcoil containing uncured lacquer are subjected to a facing-up step priorto the curing step.

2. longitudinally slitting said foil having said dry coating to producea ribbon having cut longitudinal edges, Said coating extending to saidedges;
 2. A process as claimed in claim 1, in which the end faces of thewound coil containing uncured lacquer are subjected to a facing-up stepprior to the curing step.
 3. uniformly winding said ribbon under tensionto form a coil with closely superposed edges;
 4. attaching electricalterminations to said coil; and
 5. subjecting said coil to controlledheat; whereby sufficient of said lacquer is exuded from between saidturns on melting to produce continuous end face insulation for the coilwhile leaving adequate lacquer for inter-turn insulation, said heatingbeing continued to cure said lacquer.